Sulfur dioxide is used as an intermediate in a number of different applications, including sulphonation, the generation of sulfuric acid, and to produce sulfur trioxide in electrostatic flue gas conditioning systems which use sulfur trioxide as a flue gas conditioning agent. Electrostatic flue gas conditioning systems are used to condition the exhaust flue gas of coal burning systems, such as coal fired electric generating systems, to enhance the efficiency of the electrostatic precipitator in removing particulate matter, such as fly ash, from the flue gas. Typically, in an electrostatic flue gas conditioning system, elemental sulfur is combusted or burned to generate sulfur dioxide (SO.sub.2). The SO.sub.2 is then catalyzed to convert it into sulfur trioxide (SO.sub.3). The SO.sub.3 is then injected into an electrostatic precipitator to condition the flue gas passing therethrough to enhance the efficiency of the electrostatic precipitator in removing particulate matter from the flue gas. Such a SO.sub.3 flue gas conditioning system is disclosed in U.S. Pat. No. 5,032,154.
Heretofore, elemental sulfur has been used as a source of sulfur which is combusted to generate SO.sub.2. Elemental sulfur is sulfur in its molten state. While it is inexpensive and does not readily burn, it has a number of characteristics which make it difficult to handle and store.
Elemental sulfur is delivered molten at about 280.degree. F. and must be kept at or near this temperature for successful pumping and handling. The viscosity of sulfur varies greatly with temperature. Below about 260.degree. F. the viscosity of sulfur increases quickly so that it can no longer be pumped by conventional means. Above about 300.degree. F., sulfur polymerizes into a toothpaste-like consistency and again cannot be pumped by conventional means. Elemental sulfur also has trace amounts of hydrogen sulfide which must be vented to atmosphere. Elemental sulfur also sublimates (changes from a solid to a gas and back to a solid) so that all elemental sulfur storage equipment must be steam jacketed to prevent sulfur crystal accumulations.
For these reasons, elemental sulfur storage and handling systems must be carefully designed to keep the elemental sulfur molten by keeping it within a very narrow temperature range of 270.degree. F.-290.degree. F. SO.sub.3 flue gas conditioning systems which use elemental sulfur as the feedstock typically store the molten elemental sulfur in insulated steel tanks or concrete pits. This storage vessel is heated, usually by steam coils installed in the bottom of the storage vessel. The steam coils are typically formed in a U-shape so condensate is formed in the coils as the steam cools. Thus, the elemental sulfur storage vessel must have provisions for a steam supply and for condensate disposal to a drainage facility.
Elemental sulfur storage vessels are also exposed to attack from small quantities of sulfuric acid which forms on the surface of the sulfur. Although rare, this occasionally necessitates repairs which are costly, time consuming, and carry the risk of fire.
When the elemental sulfur is pumped from the storage vessel to the sulfur furnace, where it is combusted to form SO.sub.2, its temperature must be kept within the above described narrow range of 270.degree. F.-290.degree. F. Consequently, the elemental sulfur is typically pumped through steam jacketed piping with close temperature control maintained. To maintain the elemental sulfur at the proper temperature throughout the steam jacketed piping, steam must typically be introduced at several points and condensate must also be drained from several points. Steam jacketed sulfur piping lines must also allow for pipe expansion. As a result, steam jacketed sulfur piping lines are expensive.
Elemental sulfur is unloaded into the storage vessel from a truck or rail car by the use of steam jacketed pumps which often include steam jacketed hoses. Further, although delivered "molten," the elemental sulfur often has cooled to the point where it is too viscous to be pumped properly. Steam must therefore be made available to the truck or rail car for heating the elemental sulfur to proper pumping temperatures as well as for the steam jacketed pumps and hoses used to unload the elemental sulfur.
The elemental sulfur is pumped from the storage vessel to the sulfur furnace by steam jacketed reciprocating pumps or submerged gear pumps. The reciprocating pumps require piping with check valves to prevent sulfur from flowing back during return strokes of the pump. These pumps also tend to leak because sulfur flows from all but the tightest pump joints. Hydrocarbons in the sulfur also tend to the clog the pumps and check valves. The pumping systems thus require significant maintenance on a periodic basis and tend to be the major maintenance item in sulfur based flue gas conditioning systems. Submerged gear pump assemblies have the gear pump submerged in the elemental sulfur and eliminate much of the maintenance problem. However, submerged gear pumps also require periodic maintenance.
The temperature of the various components of the elemental sulfur feedstock system must be carefully maintained to prevent minor temperature fluctuations which would quickly stop sulfur flow. As mentioned, such temperature control is achieved by the use of steam heating coils, steam pumps, and steam jacketed piping.
Although the amount of steam required by such elemental sulfur feedstock systems is relatively small, in the order of fifty to four hundred pounds per hour, saturated steam is usually not available in electric power generating plants. Consequently, the steam must be obtained by de-superheating high quality steam from turbine bleed systems or by the use of a separate boiler. In either case, such system steam, and often the condensate, is expensive and one of the major cost factors that is evaluated by a potential user of a SO.sub.3 flue gas conditioning system in deciding whether to install such a system.
It is therefore an objective of this invention to eliminate the disadvantages attendant with the use of an elemental sulfur feedstock system, such as are used in SO.sub.3 flue gas conditioning systems, by using granulated sulfur or emulsoid sulfur as the feedstock for the sulfur which is combusted to generate SO.sub.2.